Optical communications sytems, as presently contemplated, use a light source and a photodetector that are optically coupled to each other through a glass transmission line which is commonly termed an optical fiber. The light source is typically either a double heterostructure semiconductor laser or light emitting diode which have become the light sources of choice for optical communications systems. Such light source have a narrow bandgap region surrounded, on opposed major surfaces, by two wide bandgap layers. The narrow bandgap region, termed the active region, has a refractive index greater than that of the wide bandgap layers, which are termed the cladding layers. Electrons and holes combine in the narrow bandgap region and thereby generate radiation. The narrow bandgap region forms the optical cavity in which most of the light is confined. Light is used to mean electromagnetic energy in the ultraviolet, visible and infrared regions of the frequency spectrum. Due to the distribution of the density of states as a function of energy, the emission lines of semiconductor lasers are relatively broad, as compared to the emission lines of, e.g. He-Ne lasers.
A typical optical fiber has dispersive characteristics; i.e., light at different wavelengths or in different modes propagates through the fiber at different velocities. Dispersive characteristics create a problem in optical communications systems because a combination of linewidth, transmission distance, and data transmission rate can result in adjacent optical pulses overlapping, when detected, due to dispersion. Of course, information is lost when pulses overlap. The problem associated with modal dispersion can be eliminated by using single mode optical fibers, but wavelength dispersion frequently causes a problem due to the broad emission lines of semiconductor lasers. Some solutions are easily devised. For example, if the data transmission rate is decreased, the time between pulses is increased and the pulse overlap can be eliminated. This is undesirable because the data transmission rate is decreased. Other solutions, such as decreasing the transmission distance, are similarly undesirable. For this reason, as well as for other reasons, narrow linewidth lasers have been sought.
One approach users lasers with feedback means formed by, e.g. a grating. The first laser using integral feedback means is described in U.S. Pat. No. 3,760,292 issued on Sept. 18, 1973 to Kogelnik and Shank. Periodic spatial variations of, e.g. the refractive index, were used to create a single frequency output. Later developments explicitly extended the work of Kogelnik and Shank to semiconductor lasers. Such semiconductor lasers used a grating, which is optically coupled to and in or near the active layer, to obtain single frequency operation. These lasers are now termed, e.g. distributed feedback lasers. This term is, of course, applicable to lasers other than semiconductor lasers. Distributed feedback lasers are usually referred to by the acronym DFB and, in their semiconductor embodiments, have linewidths sufficiently narrow so that they are perfectly adequate for many optical communications purposes. However, as initially fabricated, they have a degeneracy which can result in emission in either one of two closely spaced emission lines. For many purposes, however, only a single emission line is desired, and this degeneracy should be removed.
Accordingly, those skilled in the art have attempted to fabricate distributed feedback lasers lasers without this degeneracy. One approach modifies the grating by introducing a quarter-wave phase shift, also termed a phase slip, which may be either abrupt or gradual. In an ideal laser, the phase slip occurs in as small portion of the total cavity as possible, thereby increasing the grating length in the two phase shifted parts of the cavity. In an abrupt phase shift laser, the gain margin, all other considerations being equal, is greater than it is for a gradual phase shift laser.
An example of an abrupt phase shift is described in U.S. Pat. No. 4,740,987 issued on Apr. 26, 1988 to McCall and Platzman. The phase slip discontinuity was located off-center, i.e., it was not located at the center of the longitudinal axis of the grating as it was in the prior art. The location of the localized phase slip was chosen by McCall and Platzman to maximize the difference between threshold gain of the lowest mode and cavity loss of the next lowest mode.
Another example of a phase shift is described in IEEE Photonics Technology Letters, 1, pp. 200-201, August 1989. The phase shift was introduced by what the authors termed a corrugation pitch modulation; i.e., the pitch of the grating was changed in one section to obtain the desired phase shift. Another approach alters the refractive index in a portion of the lasing cavity to produce the desired phase shift by, e.g., changing the transverse dimension of the active layer. This approach uses a uniform grating.
As might be expected, different techniques have been proposed for fabricating the grating. Typical techniques use laser beam holography. Another approach is described in Applied Physics Letters, 55, pp. 414-417, Jul. 31, 1989. This approach transfers a mechanically ruled grating to the semiconductor using photolithographic techniques.
However, these approaches all suffer from one or more drawbacks. An abrupt phase shift, the most technologically sophisticated approach, is difficult to implement in practice, especially if crystal growth over the grating is required. The holographic approach generates a non-uniform grating whereas, for many purposes, it would be desirable to produce a uniform grating. Variations in the refractive index produced by, e.g., changing the transverse dimension of the lasing cavity may be difficult to implement. More fundamentally, such an approach may introduce higher order modes and reflections which are undesirable. A gradual phase shift may lose significant distributed feedback action in the phase shift section, and also the gain margin is expected to be less than it is for lasers with an abrupt phase shift.